A recent study highlights (yet again) the critical importance of vultures and condors as nature’s clean up crew
Vultures and condors are seriously misunderstood birds. Because they feed on dead animals, at least some of which died from disease, people often believe they spread disease. But do obligate scavengers (vultures and condors) spread or prevent the spread of infectious diseases to other animals, including humans? Inspired by the coronavirus pandemic, an international team of scientists led by Pablo Plaza, a biologist at the National University of Comahue, recently investigated this question. Dr Plaza researches the conservation and medicine of scavenging birds.
Dr Plaza and his collaborators reviewed 76 published scientific studies of vultures and condors that live around the world and the sorts of microbes that were present in their bodies. It has been suggested that scavenging birds are resistant to infection by disease-causing microbes — viruses, bacteria or mycotic microorganisms — in their diet (ref & ref). But there are no exhaustive reviews of the health status of vultures and condors with regards to diseases nor whether they may spread these pathogens to other animals or to humans.
In their analysis, Dr Plaza and his collaborators found that vultures and condors could be colonized by a variety of microbes, some of which were resistant to multiple antibiotics. However, vultures and condors do have a variety of physiological adaptations to deal with pathogens in their diets, including extremely corrosive stomach acid and a stable intestinal microbiome, both of which could prevent ingested microbes from becoming established in their own gastrointestinal tracts.
Their feeding habits were important. Dr Plaza and his collaborators point out in their study that the speed at which obligate scavengers locate and clean up decomposing carcasses outweighs their potential as disease vectors. For example, vultures can strip a carcass clean in minutes whereas feral dogs and rats — known to spread disease — can take days to find and to clean the flesh from a dead body, which allows infectious diseases to multiply and to contaminate the environment.
On the other hand, a large carcass, like an elephant, takes longer for vultures to clean up, which provides the opportunity for pathogens to multiply and contaminate the environment. In this situation, vultures may potentially act as disease vectors. But this was rare and there was no clear evidence that vultures play much of a role in spreading pathogens to humans and other species.
Tragically for vultures and condors, we do not understand or appreciate them at all. Our disgust at their critically important ecological role as nature’s “clean up crew” by feeding on dead animals has led to persecution and shooting, trading their body parts for traditional “medicine”, and accidental and intentional poisoning with a variety of substances (ref). For example, in India, there was an astounding decline of 97-99.9% of its three vulture species between 1992 and 2007 due to ingestion of dead cattle treated with the pain reliever, diclofenac, which is extremely toxic to vultures. This sudden and dramatic plunge in vulture numbers led to the rapid spread of zoonotic diseases, especially an increased incidence of rabies.
In summary, this analysis did not unearth any clear evidence that vultures and condors spread pathogenic microbes and antibiotic resistance to other animals — probably because this is an exceedingly rare event. It did remind us that they provide valuable ecosystem services by removing dead animals from the environment, thereby limiting the spread of pathogens. But how they can consume diseased flesh without becoming diseased themselves is poorly understood and certainly requires more study. By highlighting that vultures are critically important for dealing with dead animals, Dr Plaza and his collaborators hope to clean up their image and promote their conservation.
“Further research should evaluate the potential of vultures in disease regulation to avoid misconceptions and to promote scientific evidence of the ecosystem service they provide”, Dr Plaza said in a press release. “This will help to conserve this globally threatened avian group and maintain the contributions they provide to people.”
Pablo I. Plaza, Guillermo Blanco and Sergio A. Lambertucci (2020). Review: Implications of bacterial, viral and mycotic microorganisms in vultures for wildlife conservation, ecosystem services and public health, Ibis, published online on 13 August 2020 ahead of print | doi:10.1111/ibi.12865